Add `flox include list` command

[mkenigs]

Included environments can be renamed in the manifest, or they can pull in the name of the included environment, which may or may not match that of the directory that contains it. Because of that, it's not immediately obvious what an included environment's name is. But a user needs to know the name in order to run flox include upgrade name. (an alternative way to make that easier is allow specifying directory names instead of just names)

Add a command to print included environment's names, as well as anything else we think would be helpful.